Flux-gate magnetometers have significant advantages in size, weight, power consumption, and reliability for use in the measurement of magnetic fields, particularly small magnetic fields. Generally, a flux-gate magnetometer includes one or more sensors which include a magnetizable core and at least one coil wound around the core. The flux-gate magnetometer senses the magnetic field by stimulating the sensor with a known signal. The known signal is used to drive the core in and out of saturation. The nonlinear magnetic properties of the core cause the second harmonic of the frequency of the known drive signal to be generated. The magnitude of the external magnetic field, i.e., the magnetic field to be measured, is proportional to or can be determined as a function of the second harmonic. For example, in the absence of any component of an external magnetic field, the peaks detected in an output voltage generated across a sensor of the flux-gate magnetometer may be uniform. On the other hand, in the presence of an external magnetic field, the voltage peaks may vary in a manner which may be measured by applying the output voltage to signal conditioning circuitry so as to provide a measurement signal representative of the external magnetic field to be measured.
In other words, the measurement of the external magnetic field is performed through modulation of a core of variable permeability. The modulated field is detected with the coil wound about the core. A change in permeability is accomplished with the known drive signal, e.g., drive current, in the coil wound about the core in such a fashion as to saturate the core during part of the cycle of the drive waveform. Modulation of the magnetic field to be sensed occurs only at even harmonics of the drive waveform due to the symmetry of the magnetization curve. Generally, the second harmonic is used as the measure of the external magnetic field.
Problems may occur in flux-gate magnetometers if operated in high noise environments, e.g., such as in an automobile or around other noisy equipment producing EMI. For example, if the frequency of the EMI is twice that of the drive signal, i.e., equal to the second harmonic frequency, the EMI will be undesirably sensed by the sensor(s) and interpreted as all or a part of an external magnetic field.
In the past, differential circuitry has been used to reduce the effects of EMI. For example, such differential techniques may involve the use of two sensors oriented opposite to one another in a magnetic field to be measured such that one sensor would provide a second harmonic signal which is inverted with respect to the other sensor. Using subtraction of the two signals, noise which is common to both of the sensors (i.e., common mode noise) can be canceled. However, in high noise environments, such differential circuit techniques do not provide adequate EMI immunity.
Conventionally, the drive signal used for driving the core in and out of saturation is a periodic and repetitive drive signal. For example, drive signals which have been used in the past to drive the sensors in and out of saturation include repetitive and periodic triangular waveforms and other repetitive and periodic waveforms, such as those waveforms having a constant duty cycle and/or a constant frequency over time. Flux-gate magnetometers are well known in the art, some examples of which may be found in the issued patents and references listed in Table 1 below.
TABLE 1 Patent No. Inventor(s) Issue Date Articles 3,638,074 Inouye 25 January 1972 "Sensor Noise in 5,530,349 Lopez, et al. 25 June 1996 Low-Level Flux-Gate Magnetometers," by D.C. Scouten, IEEE Transactions on Magnetics, Vol. Mag-8, No. 2, (June 1972)
All references listed in Table 1 above are herein incorporated by reference in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, the Detailed Description of the Embodiments, and the claims set forth below, any of the devices or methods disclosed in the references of Table 1 may be modified advantageously by using the teachings of the present invention.